Molecular Geometry · VSEPR

VSEPR

The valence electron pairs around a central atom (bonding pairs and lone pairs) are negatively charged, so they repel one another and move as far apart as possible.

On the left, every electron domain is pushed across a sphere by all the others. Add or remove pairs, or click Shake, and watch them spring apart into the most stable arrangement.

A lone pair is held by only one nucleus, so it sits closer to it and spreads wider. That makes its repulsion stronger.

Electron vs. molecular geometry

The central atoms of CH₄, NH₃ and H₂O all have 4 pairs of valence electrons, so all three have a tetrahedral electron geometry (the green polyhedron).

Molecular geometry counts only the atoms. A lone pair occupies a vertex but is “invisible”, so NH₃ is trigonal pyramidal and H₂O is bent.

Bond angles

Repulsion strength: lone–lone > lone–bond > bond–bond.

The more lone pairs, the closer the bonding pairs are squeezed and the smaller the bond angle:

CH₄ 109.5° > NH₃ 107° > H₂O 104.5°

The arcs are labelled with measured bond angles. The shapes come from the repulsion model, which follows the same trend as experiment.

Hybridization

The central atom “mixes” atomic orbitals of similar energy into the same number of equivalent hybrid orbitals. Each hybrid orbital holds one electron domain.

Number of hybrid orbitals = number of valence electron pairs: 2 → sp, 3 → sp², 4 → sp³, 5 → sp³d, 6 → sp³d².

In ethylene, acetylene and benzene, the unhybridized p orbitals form π bonds.

Polarity

Every polar bond has a bond dipole (thin arrow, pointing to the more electronegative end).

Add them as vectors. If the net dipole is zero the molecule is nonpolar; otherwise it is polar (thick arrow μ).

Both have three atoms: linear CO₂ cancels its dipoles, while bent SO₂ does not.

Expanded octets

Elements from period 3 onward can have 5, 6 or even 7 electron domains.

In a trigonal bipyramid an equatorial site has only two 90° neighbours while an axial site has three, so lone pairs fill equatorial sites first: SF₄ is a seesaw, ClF₃ is T-shaped and XeF₂ is linear.

In an octahedron two lone pairs sit trans to each other, so XeF₄ is square planar.

Chirality

When a carbon atom carries four different atoms or groups, the molecule and its mirror image are like left and right hands: no rotation can superimpose them.

Click Superimpose to turn the mirror image onto the original and see which atoms fail to match.

Library

Browse every molecule by VSEPR class. Drag to rotate and scroll to zoom.